Device and method for controlling a gas discharge lamp, and lighting system with gas discharge lamp and control device

ABSTRACT

A device and a method for the control of a gas discharge lamp are disclosed. In order to detract as little as possible from lamp life in spite of the luminous flux requirements to be fulfilled during the run-up of the lamp, the lamp is operated with an alternating current in a run-up phase which comprises at least the interval from 1 s to 3 s after lamp ignition, the amplitude of said current rising during the run-up phase. After the rise in the run-up phase, the current may first rise further or remain constant in a transitional phase which preferably follows the former phase, and is subsequently reduced until the lamp enters the stationary operational phase. The time gradient of the current is preferably chosen such here that minimum values for the luminous flux of the lamp are achieved at given moments. Particular advantages are obtained, for example, in the case of Hg-free lamps which are operated with high currents, especially during the run-up.

The invention relates to a device for controlling a gas discharge lamp,and a lighting system with a gas discharge lamp and a control device, aswell as to a method of controlling a gas discharge lamp.

Light is generated by means of a gas discharge in gas discharge lamps,which discharge usually takes place in a discharge vessel between twoelectrodes. The gas discharge is ignited in that an ignition voltage isapplied, which leads to the formation of a light arc. After ignition ofthe light arc, the electrodes and the vessel surrounding them are heatedup, and the lamp enters its stationary operational state (or steadystate) after some time.

Discharge lamps have been widely used in particular in the automotivefield for some time. The lamps used here are usually operated at an ACvoltage in stationary operation.

The sequence in time after ignition of a discharge lamp is as follows:after ignition of the light arc, the lamp is first operated in atransitional state for a few milliseconds, usually 100 ms or less. Inthis transitional state, the lamp is operated with a DC voltage, whosepolarity may be reversed, however, for example twice. This serves toheat up the electrodes. At the end of the transitional state, the lampis operated with an AC voltage, often a square-wave AC voltage of 400Hz. This comprises an initial interval of a few seconds, which isfollowed by stationary operation in the steady state.

A rise in the luminous flux of the lamp which is as fast as possible isusually aimed at, in particular for the use in the automotive field. Itis known for this purpose to operate the lamp with a constant, very highrun-up current in an initial phase, which current is close to a maximumadmissible current for the respective lamp, taking into account adesired lamp life.

U.S. Pat. No. 5,663,875 describes a voltage converter for operating adischarge lamp. FIG. 1 b herein represents the current gradient afterignition of the lamp. In the transitional range, this current isinitially raised to a very high value, still in DC operation, and isthen switched to AC operation, in which initially a very strongalternating current flows which is then slowly reduced down to nominaloperation. The lamp, which has a rated power of 35 W, is operated withup to 90 W in the initial period so as to heat up the electrodes and toevaporate the ingredients present in the discharge vessel.

U.S. Pat. No. 5,434,474 also relates to a device for operating adischarge lamp. The device comprises a circuit for detecting excesscurrents so as to avoid high run-up currents. In the initial period, thedevice limits the current to a maximum value. The object is to avoidadverse consequences for lamp life.

It is an object of the invention to provide a method and a device forthe control of a gas discharge lamp as well as a lighting system with agas discharge lamp and a corresponding control device in which the lampis controlled such that its life is not unnecessarily shortened, whilenevertheless the lamp complies with the requirements relating to itsrun-up behavior.

This object is achieved by means of a device as claimed in claim 1, alighting system as claimed in claim 9, and a method as claimed in claim11. Dependent claims relate to advantageous embodiments of theinvention.

The invention is based on the recognition that the operation with astrong current is extremely disadvantageous for lamp life especially inthe cold state of the lamp after ignition because of the thermalexpansion taking place then. The requirements relating to the run-up ofthe lamp are defined, for example, by set values for the lamp luminousflux at several moments. Thus a lamp must achieve at least a firstthreshold value for its luminous flux, for example after 1 s, so as tocomply with these specifications, and at least a second, higherthreshold value, for example after 4 s.

According to the invention, the lamp is driven with an AC current ofessentially rising amplitude in a run-up phase by means of a currentsupply device. Instead of operating the lamp right from the start at themaximum admissible current, as is known from the prior art, a timeperiod of essentially rising amplitude of the AC current flowing throughthe lamp is provided at least in the time interval of between 1 and 3 safter lamp ignition. The term “essentially” rising is to be understoodhere in the sense that the value is lower at the start of the run-upphase than at the end of the run-up phase. It is proposed in a furtherembodiment that the time gradient of the current in the run-up phase ismonotonically rising viewed over time, i.e. it rises or remains constantin some time sections, but does not drop. A means which is suitablysmoothing over time may have to be provided for this because of theundular character of the current.

The concept “run-up phase” is used here for any time interval in whichthe lamp is operated with an alternating current, which intervalcomprises at least the time period from 1 s to 3 s after lamp ignition.The run-up phase may then start immediately after a (DC) transitionalphase. The start of the run-up phase may also take place in a period inwhich the lamp is already being operated with an alternating current.Depending on the application, the run-up phase may be expanded in theone and/or other direction in time and start, for example, as early as0.5 s, 0.3 s, or even earlier after ignition and end, for example, after4, 5, or even as many as 8 s after ignition.

It is provided in an advantageous further embodiment that the currentrises by at least 30% in the run-up phase with respect to the value atthe beginning of said phase. Preferably, however, a rise of more than50%, in some cases even above 100%, is chosen.

In a further embodiment, the current reaches a maximum value in therun-up phase or in the transitional phase which preferably follows it.This maximum value is preferably determined for the respective lamp typesuch that minimum requirements as regards lamp life are complied with.In a further embodiment of the invention, the amplitude of the currentat the start of the run-up phase is at most 75% of the maximum current,preferably less than 60% thereof.

The given gradient may be, for example, linear in the form of a risingslope, or any other rising curve shape, for example stepped, etc. Thegradient in time in any concrete application is preferably laid down onthe basis of experiences with the lamp type used. It may be readilyascertained in tests which rise of the current gradient suffices forachieving the given minimum values for the lamp luminous flux at givenmoments. It generally suffices when the envisaged values are justachieved, possibly with a certain safety margin. A clear overshoot mayhave negative consequences for lamp life.

The run-up phase is preferably followed by a transitional phase, forexample of a few seconds, in which the amplitude of the current remainsconstant, for example, and finally drops to the value which it hasduring stationary operation. It is alternatively possible, however, forthe current rise to continue initially also in the transitional phase.

In a preferred embodiment, the control device comprises amicrocontroller or microprocessor which provides a previously storedprogrammed time gradient for the run-up current to a controllablecurrent supply device. The previously defined time gradient here isstored in the microcontroller, for example in the form of a table. In afurther embodiment of the invention, the microcontroller also monitorsthe operational state of the lamp, i.e. it can decide during ignition ofthe lamp whether a cold ignition takes place or a re-ignition of a stillhot lamp. In the latter case, the microcontroller can control the lampsuch that it is operated with a substantially weaker current in therun-up phase, because the otherwise necessary heating of the lampsubstantially does not apply now.

The lighting system according to the invention comprises a gas dischargelamp with a suitable control device. The values for the current gradientin the run-up phase are preferably stored in the control device, whichvalues are necessary if the lamp in question is to fulfill therequirements.

Tests have shown that the initially “protective” rise of the currentduring the run-up phase has a positive influence on lamp life. Theprogrammed current gradient in the run-up phase at the same time ensuresthat the specifications relating to the run-up behavior are fulfilled inall cases. A clear prolongation of lamp life is observed, especially forlamps operated at high currents. For example, discharge lamps withfillings in the discharge vessels which are free from Hg, which areoperated at higher currents because of the resulting lower burningvoltage in particular during the run-up, profit from the invention as aresult.

An embodiment of the invention will be explained in more detail belowwith reference to drawings, in which:

FIG. 1 is a side elevation of a gas discharge lamp;

FIG. 2 is a diagram of the circuit of a lighting system with a controldevice and a gas discharge lamp;

FIG. 3 is a time diagram in which the gradients of the voltage across alamp and the current through a lamp in the prior art are depicted;

FIG. 4 is a time diagram in which the gradient of the current throughthe lamp in an embodiment of the invention is depicted; and

FIG. 5 is a time diagram showing the gradients of lamp current and lampluminous flux in the run-up phase in an embodiment of the invention.

FIG. 1 shows a typical gas discharge lamp 10 as it is used in theautomotive field. The lamp 10 is shown by way of example only here.Details on the construction and function of such lamps are known tothose skilled in the art. The details are accordingly not discussed anyfurther, but the most important components of the lamp 10 are merelymentioned.

A burner with a discharge vessel 14 is retained in a lamp base 12.Electrodes 16 project into the interior of the discharge vessel 14,which is closed off by a quartz wall. A gas discharge is maintainedbetween the electrodes 16 during operation of the lamp 10. The dischargevessel 14 contains a filling free from Hg in the example shown. The lamp10 is operated at a power of 35 W during stationary operation with acurrent of approximately 830 mA and at a voltage of 42 V.

FIG. 2 shows a diagram of a lighting system 20 comprising a controlcircuit 22 and the lamp 10.

The control circuit 22 comprises a controllable current supply 24 whichis controlled by a microcontroller μC. A current sensor 26 and a voltagemeasuring device 28 measure the current through and the voltage acrossthe lamp 10, supplying the results to the microcontroller μC.

FIG. 3 shows the time gradient of the voltage drop U across a dischargelamp and of the current I_(L) through a discharge lamp in accordancewith the prior art. The representation in FIG. 3 is purely diagrammatichere and merely serves to clarify the principle of the time gradientduring lamp ignition.

The voltage U is increased for igniting the lamp to the point where alight arc is ignited at moment t=0. The ignition of the light arc isfollowed by the transitional phase A in which the lamp is operated witha direct current (DC voltage). The transitional phase A shown by way ofexample lasts for a few milliseconds, up to a maximum of 100 ms. Thepolarity is changed once during this period in the example shown.

The transitional phase A is followed by an initial phase B in which thelamp is operated with an alternating current (AC voltage). The initialphase B lasts for a few seconds and serves to “run up” the lamp. In theprior art as shown in FIG. 3, the lamp is operated immediately with themaximum admissible current in the initial phase B, which current is thenreduced gradually to the stationary phase C after a successful run-up.

Lamp operation is by means of a square-wave AC voltage with a frequencyof 400 Hz both in the initial phase B and in the stationary phase C. Itshould be pointed out once more that the gradient shown in FIG. 3 isindicated symbolically only, and accordingly, for example, the number ofpolarity changes shown in the interval B cannot be used for drawingconclusions on the duration of this interval.

FIG. 4 shows the time gradient of the current I_(L) through the lamp inan embodiment of the invention. This again is a purely symbolic picturedesigned for clarifying the difference with the prior art of FIG. 3.

In the time diagram of FIG. 4, the ignition at moment t=0 is againfollowed by a transitional phase A of a few milliseconds during whichthe lamp is operated with a direct current (DC voltage). This isfollowed by a run-up phase B1 in which the lamp is operated with analternating current. The run-up phase B1 has a duration of a fewseconds. A rise in amplitude of the current I_(L) takes place in thisrun-up phase, so that the value at the end of the phase is higher thanat the beginning of the phase.

The run-up phase B1 is followed by the transitional phase B2 in whichthe increased current obtained in the run-up phase B1 is maintained forsome time in this example and finally drops to the value which itassumes in the phase C of stationary operation. The phase B2 has aduration of a few seconds.

In contrast to the prior art, the high run-up current is not suddenlyswitched on, but a rise in amplitude of the current is provided. Thismeans that the lamp is loaded considerably less strongly during theperiod B1 of a few seconds, which has a positive influence on lamp life.The current integral in the phase B1 has a clearly lower value than inthe corresponding interval in the prior art.

FIG. 5 shows an example of the gradient of the current I_(L) in therun-up phase for the lamp 10. The number of seconds elapsed since lampignition at moment t=0 s is plotted on the time axis. The curve I_(L)shows the gradient of the current through the lamp 10. The curve L showsthe gradient of the luminous flux of the lamp. The stepped curve L_(D)indicates the minimum luminous flux required for this case. For example,European standard ECE R 99 stipulates that 25% of the nominal luminousflux should be achieved 1 s after lamp ignition, and 80% of the luminousflux 4 s after starting.

In the example shown, the phase in which the lamp is operated with asquare-wave AC voltage of 400 Hz starts at approximately t=0.1 s.

In the run-up current phase B in the prior art (FIG. 3), the amplitudeof the current I_(L) is first set for the maximum value admissible forthe respective lamp type so as to achieve a run-up of the lamp and arise of the luminous flux of the lamp which are as fast as possible. Thevalue of the current I_(L) is subsequently reduced continuously down tothe level of phase C, in which the lamp is in stationary operation.

In contrast to this, FIG. 5 shows a rising, for example essentiallyrising time gradient of the current I_(L). As is apparent from FIG. 5,the curve of the current I_(L) here is not smooth, but exhibits astrong, irregular oscillation about a time average. As regards thegradient of the current I_(L), the embodiments accordingly relate to asuitably smoothed, sliding average over time. In the interval from 1 sto 3 s after ignition, the current rises from approximately 2.25 A toapproximately 3.5 A, i.e. by approximately 55%. The gradient ismonotonically rising averaged over time, also remaining constant incertain regions (for example from approximately 1 s to approximately 1.5s). I_(L) rises from approximately 1.75 A to approximately 3.75 A in theinterval from t=0.5 s to t=4 s, i.e. by more than 100%. The twointervals mentioned above are examples for the “run-up phase” B1.

As the curve L in FIG. 5 shows, the chosen gradient of the rise of I_(L)in the run-up phase has the result that the luminous flux of the lamp(L) has a gradient wherein the minimum values at t=1 s and t=4 s inaccordance with the requirement L_(D) are achieved without anunnecessarily high overshoot of the requirements taking place.

Those skilled in the art may determine the necessary gradient for I_(L)for each lamp or for each lamp type such that the luminous flux of thelamp L complies with the relevant requirements. This may take placeexperimentally in a simple manner.

In the gradient for I_(L) shown in FIG. 5, the maximum current I_(L) isapproximately 3.75 A. This corresponds to the maximum current admissiblefor the lamp, taking into account a required minimum lamp life. Thismaximum current, however, is not achieved until after approximately 3.25s after lamp ignition, in contrast to the prior art. The lamp isinitially run up in a “protective” manner in the preceding run-upinterval, which leads to a substantial lengthening of lamp life, astests have demonstrated.

The gradient of the current I_(L) shown in FIG. 5 is achieved with thelighting system 20 of FIG. 2 in the following manner.

When the microprocessor μC receives a start signal S for starting thelamp, the controllable current supply 24 is initially controlled suchthat the lamp 10 is ignited. Additional circuits for generating theignition voltage (not shown) known to those skilled in the art may beused for this.

After lamp ignition and after the transitional phase A, themicrocontroller μC controls the current supply 24 such that analternating current with the gradient as shown in FIG. 5 is generated.The gradient of I_(L) necessary for complying with the specificationsI_(D) was previously calculated. The time gradient of I_(L) necessaryfor this is stored in the microcontroller μC in the form of a tablecomprising the respective values of I_(L) at various moments. Themicrocontroller μC controls the current supply 24 in accordance withthese stored values such that I_(L) is given the gradient as shown inFIG. 5.

The invention may be summarized in that a device and a method for thecontrol of a gas discharge lamp are disclosed. In order to detract aslittle as possible from lamp life in spite of the luminous fluxrequirements to be fulfilled during the run-up of the lamp, the lamp isoperated with an alternating current in a run-up phase which comprisesat least the interval from 1 s to 3 s after lamp ignition, the amplitudeof said current rising during the run-up phase. After the rise in therun-up phase, the current may first rise further or remain constant in atransitional phase which preferably follows the former phase, and issubsequently reduced until the lamp enters the stationary operationalphase. The time gradient of the current is preferably chosen such herethat minimum values for the luminous flux of the lamp are achieved atgiven moments. Particular advantages are obtained, for example, in thecase of Hg-free lamps which are operated with high currents, especiallyduring the run-up.

1. A device for controlling a gas discharge lamp (10) with a currentsupply device (24) for supplying the lamp (10) with an alternatingcurrent (IL) of given amplitude, and a programming unit (μC) forproviding amplitude values to the current supply device (24) during arun-up phase (B1), wherein the run-up phase comprises at least theinterval from 1 s after ignition of the lamp (10) to 3 s after ignitionof the lamp (10), and wherein the programming unit (μC) effectuates asubstantially rising gradient in time of the current (IL) during therun-up phase (B1).
 2. A device as claimed in claim 1, wherein the timegradient is chosen such that the luminous flux (L) generated by the lamp(10) achieves at least at two given moments assigned minimum valves. 3.A device as claimed in claim 1, wherein the run-up phase (B1) comprisesat least the interval from 0.5 s after ignition of the lamp (10) to 4 safter ignition of the lamp (10).
 4. A device as claimed in claim 1,wherein the current (IL) rises by at least 30% in the run-up phase (B1)with respect to the value at the start of said phase.
 5. A device asclaimed in claim 1, wherein the time gradient of the current (IL) in therun-up phase (B1) rises monotonically averaged over time.
 6. A device asclaimed in claim 1, wherein the current (IL) is an alternating currentwith a substantially square-wave characteristic in time and a frequencyof at least 200 Hz.
 7. A device as claimed in claim 1, wherein thecurrent (IL) drops to a stationary value in a transition phase (B2)following the run-up phase (B1).
 8. A device as claimed in claim 1,wherein the current (IL) at the start of the run-up phase (B1) amountsto at most 75%, preferably less than 60% of the maximum value that thecurrent assumes in the interval after 1 s after ignition.
 9. A lightingsystem with a gas discharge lamp (10) and a control device (22) asclaimed in any one of the claims 1 to
 8. 10. A lighting system asclaimed in claim 9, wherein the gas discharge lamp (10) has a fillingfree from Hg.
 11. A method of controlling a gas discharge lamp whereinan alternating current (IL) flows through the lamp in a run-up phase(B1) which comprises at least the interval from 1 s after ignition of alamp (10) to 3 s after ignition of the lamp (10), wherein the current(IL) is controlled such that its amplitude rises during said run-upphase, and wherein the time gradient of the current (IL) is chosen suchthat the luminous flux (L) generated by the lamp (10) achieves at atleast to two given moments in time (B1) assigned minimum valves.